In radio-frequency (RF) communications, multiple-input-multiple-output (MIMO) refers to using multiple antennas at both the transmitter and the receiver ends to enhance communication performance. The terms input and output in MIMO denote the radio channels that carry the input and output signals received by or transmitted from a communication device (e.g., a transceiver). MIMO, which is an important portion of modern wireless communication standards such as IEEE 802.11n (e.g., Wi-Fi), 4G, 3GPP Long Term Evolution (LTE), IEEE 802.16e (e.g., WiMAX), and others, can significantly increase data throughput and link range without the need for additional bandwidth or increased transmission power. Three main categories of MIMO include spatial multiplexing, diversity coding, and precoding.
In precoding, multiple streams of signals are generated by beamforming. This is in contrast with the conventional single-stream beamforming, where the same signal is transmitted by each of the multiple antennas with independent and appropriate phase and gain weightings, such that the signal power is maximized at a receive end (e.g., an antenna of a multiple-antenna receiver). However, in order to simultaneously maximize the signal level at a number of receive ends (e.g., more than one antenna of a multiple-antenna receiver), multi-user (MU)-MIMO has to be used to generate multiple streams of signals. MU-MIMO, which is part of the IEEE 802.11ac standard, allows a transmitter (e.g., a router such as an access point) to simultaneously transmit data to more than one station (e.g., terminal or user equipment). Each of the multiple streams of data transmitted by the transmitter is intended for one of the stations. The beamforming vectors (e.g., gain and phase matrices) are chosen such that each of the stations receives its intended data streams at high power and receives all non-intended data streams at very low power and/or in particular spatial directions. In order to design such beamforming vectors, the MU-MIMO beamformer must have accurate knowledge of the propagation channel from itself to each of the intended recipients. Such channel knowledge is typically acquired via a feedback process, where the MU-MIMO beamformer transmits a sounding frame, and upon reception of this sounding frame each user terminal estimates the channel from the MU-MIMO beamformer to itself. In the next stage, each of the user terminals transmits its channel estimate (e.g., a quantized channel estimate) back to the MU-MIMO beamformer. Due to the time variation of the propagation channel and the need for very accurate channel knowledge at the beamformer, this process is frequently repeated, and therefore consumes considerable overhead.